The present invention relates to a solid-electrolyte secondary battery having a solid electrolyte (also a gel electrolyte) disposed therein between a positive electrode and negative electrode, and more particularly, to a novel solid-electrolyte secondary battery improved in charge and discharge cycle life, volumetric energy density, load characteristic at low temperature, productivity, etc.
In recent years, many portable electronic apparatuses such as an integral VTR/video camera unit, portable telephone, portable computer, etc. have been proposed, and they show a tendency to be more and more compact for their improved portability. Many developments and researches have been made to provide a thinner or bendable battery, more specifically, a secondary battery, or a lithium ion battery among others, for use as a portable power source in such a more compact portable electronic apparatus.
To attain such a thinner or bendable battery structure, active researches have been made concerning a solidified electrolyte for use in the battery. Especially, a gel electrolyte containing a plasticizer and a polymeric solid electrolyte made from a high molecular material having lithium salt dissolved therein are attracting much attention from many fields of industry.
As the high molecular materials usable to produce a high molecular solid electrolyte, a silicone gel, acryl gel, acrylonitrile, polyphosphazen-modified polymer, polyethylene oxide, polypropylene oxide, their composite polymer, cross-linked polymer, modified polymer, etc. have been reported. In the conventional secondary battery using a solid electrolyte made from one of these high molecular materials, however, since the electrolyte film has no sufficient film strength and adhesion to the battery electrodes, there occurs a nonuniformity between the charge and discharge currents, and a lithium dendrite easily takes place. Thus, the conventional secondary battery has a short charge and discharge cycle life (number of charge and discharge cycles), namely, it is critically disadvantageous in that it cannot meet the requirement xe2x80x9cstable usability for a longer termxe2x80x9d being one of the basic and important requirements for production of a commercial article.
Further, for a higher film strength of a solid electrolyte, it has been proposed to cross-link a trifunctional polyethylene glycol and diisocyanate derivative by reaction between them (as disclosed in the Japanese Unexamined Patent Publication No. 62-48716) or to cross-link polyethylene glycol diacrylate by polymerization (as disclosed in the Japanese Unexamined Patent Publication No. 62-285954). Because an unreacted substance or a solvent used for the reaction remains, the electrolyte has no sufficient adhesion to the battery electrodes. Moreover, the indispensable process of drying removal causes the productivity to be low. These methods are required for a further improvement.
As mentioned above, the high molecular solid or gel electrolyte has excellent characteristics not found with the liquid electrolytes, but when it is used in a battery, it can hardly be put in ideal contact with the battery electrodes. This is because the solid or gel electrolyte will not flow as the liquid electrolyte.
The contact of the high molecular solid or gel electrolyte with the battery electrodes has a large influence on the battery performance. Namely, if the contact between them is poor, the contact resistance between the high molecular solid or gel electrolyte and the battery electrodes is large so that the internal resistance of the battery is large. Furthermore, there cannot be an ideal ion movement between the high molecular solid or gel electrolyte and the electrodes, and so the battery capacity is also low. If such a battery is used for a long term, there occurs a nonuniformity between the charge and discharge currents and a lithium dendrite is likely to take place.
Therefore, in a battery using a high molecular solid or gel electrolyte, it is extremely important to adhere the high molecular solid or gel electrolyte to active material layers of electrodes of the battery with a sufficient adhesive strength.
To implement the above, it has been proposed as in the Japanese Unexamined Patent Publication No. 2-40867 to use a positive electrode composite in which a high molecular solid electrolyte is added to a positive active material layer of the positive electrode. In the battery disclosed in the Japanese Unexamined Patent Publication, a part of the high molecular solid electrolyte is mixed in the positive active material layer to improve the electrical contact between the high molecular solid electrolyte and positive-electrode active material layer.
However, in case the method disclosed in the Japanese Unexamined Patent Publication No. 2-40867 is adopted, the positive-electrode composite to which the high molecular solid electrolyte is added must be used to produce a positive plate and the high molecular solid electrolyte should be laminated on the positive plate. No ideal contact can be attained between the positive plate and solid electrolyte. More specifically, if a solid electrolyte having an irregular surface is laminated on an electrode layer, no good adhesion between them can be ensured and the internal resistance will be increased, with a result that the load characteristic becomes worse. Also, a positive or negative electrode composite in which a high molecular solid or gel electrolyte is added cannot easily be pressed to a sufficient extent because of the elasticity of the high molecular solid or gel electrolyte, and the grain spacing inside the composite is large, with a result that the internal resistance is increased. Also in this case, the load characteristic becomes worse. Furthermore, to prevent an electrolyte salt contained in the high molecular solid or gel electrolyte from being dissolved, the positive or negative electrode should be produced at a low humidity, their quality cannot easily be controlled, and the manufacturing costs are large.
Also, it has been proposed to use a copolymer produced by copolymerization of 8 to 25% by weight of hexafluoroethylene with the fluorocarbon polymer in order to improve the load performance and low- temperature performance. However, the addition of the hexafluoroethylene in such an amount will lower the crystallization temperature of the polymer, thus resulting in a deteriorated film strength.
Thus, the action to isolate the positive and negative electrodes from each other is considerably decreased. If the film thickness is not as large as 100 xcexcm or so, a short-circuit will arise between the electrodes. Such a large film thickness will not provide a necessary volumetric energy density for the battery as a commercial article. Therefore to reduce the film thickness for a desired volumetric energy density, a third means for reinforcing the film strength should be used, which will add to the manufacturing labor and costs.
For the same reason, the maximum amount of an electrolyte is 70% by weight. If a large amount is added, the electrolyte cannot keep the form of a film but it will take the form of a sol. This will be the performance limit of the battery and it is difficult to ensure a sufficient load performance and low-temperature performance.
Accordingly, the present invention has an object to overcome the above-mentioned drawbacks of the prior art by providing a solid electrolyte excellent in adhesion to the active material layers of the electrodes, and thus providing a solid-electrolyte secondary battery using therein the solid electrolyte to ensure a good electrical contact between the solid electrolyte and active material layers of a positive electrode and negative electrode of the battery.
Also, the present invention has another object to provide a solid-electrolyte secondary battery having an improved charge and discharge cycle life and excellent in load characteristic, low-temperature performance and productivity.
To attain the above object, the Inventors have been made many researches for a long term. As a result of the researches, it has been found that the molecular structure of a fluorocarbon polymer used as a matrix polymer in the solid electrolyte has a great influence on the characteristics of the electrolyte, use of a vinylidene fluoride/hexafluoropropylene block copolymer makes it possible to adhere the high molecular solid or gel electrolyte with a sufficient adhesive strength to the active material layers of the electrodes, provide a good electrical contact between the solid or gel electrolyte and the active material of the positive and negative electrodes and ensure a sufficient film strength, and thus provide a solid-electrolyte secondary battery having a longer charge and discharge cycle life and excellent in load characteristic, low-temperature performance and productivity.
The solid-electrolyte secondary battery according to the present invention is completed based on the above findings by the Inventors and comprises a positive electrode and negative electrode and a solid electrolyte provided between the electrodes, the solid electrolyte containing as a matrix polymer a vinylidene fluoride/hexafluoropropylene block copolymer.
Note that the term xe2x80x9csolid electrolytexe2x80x9d used herein refers to a so-called solid electrolyte as well as to a gel electrolyte in which a matrix polymer is plasticized by a plasticizer, for example. Therefore, the solid-electrolyte secondary battery of the present invention includes a gel-electrolyte secondary battery as well.
The present invention is essentially characterized in that a vinylidene fluoride/hexafluoropropylene block copolymer is used as a matrix polymer. The block copolymer assures an excellent adhesion of the electrolyte to the active material layers of positive and negative electrodes, and the properties of the individual monomers assure a sufficient toughness and solvent retention in combination. Therefore, it is possible to adhere the high molecular solid or gel electrolyte to the active material of the electrodes with a sufficient adhesive strength, retain a large amount of solvent (electrolyte) while maintaining a high film strength, and implement an improved charge and discharge cycle life, load characteristic and low-temperature performance.
In an embodiment, the present invention provides a solid-electrolyte secondary battery that comprises a positive electrode, a negative electrode and a solid electrolyte disposed between the positive and negative electrodes. The solid electrolyte comprises a vinylidene fluoride/hexafluoropropylene block copolymer as a matrix polymer.
In an embodiment, the vinylidene fluoride/hexafluoropropylene block copolymer comprises from about 3% to about 7.5% by weight hexafluoropropylene.
In an embodiment, the vinylidene fluoride/hexafluoropropylene block copolymer has a weight-average molecular weight of greater than 550,000.
In an embodiment, the vinylidene fluoride/hexafluoropropylene block copolymer comprises first components having a weight-average molecular weight of greater than 550,000, second components having a weight-average molecular weight of greater than 300,000 and third components having a weight-average molecular weight of less than 550,000.
In an embodiment, the solid electrolyte further comprises an electrolyte in an amount greater than 80% by weight.
In an embodiment, the negative electrode comprises a material into which a lithium ion can be inserted or from which a lithium ion can be extracted.
In an embodiment, the material into which a lithium ion can be inserted or from which a lithium ion can be extracted comprises a carbon material.
In an embodiment, the positive electrode comprises a composite oxide of lithium and a transition metal.
In an embodiment, the positive electrode comprises a face and the negative electrode comprises a face. The faces of the positive and negative electrodes are spaced apart from one another, the solid electrolyte sandwiched therebetween. The solid-electrolyte secondary battery further comprises a solution in which a solid electrolyte is dissolved and which impregnates the faces of the positive and negative electrodes.
These objects and other objects, features and advantages of the present intention will become more apparent from the following detailed description of the preferred embodiments of the present invention when taken in conjunction with the accompanying drawings.